All-solid secondary battery and method of preparing the same

ABSTRACT

An all-solid secondary battery, including: a cathode; an anode; and a solid electrolyte layer disposed between the cathode and the anode, wherein the anode comprises an anode current collector; a first anode active material layer in contact with the anode current collector and comprising a first metal; a second anode active material layer disposed between the first anode active material layer and the solid electrolyte layer and comprising a carbon-containing active material; and a contact layer between the second anode active material layer and the solid electrolyte layer, and disposed such that the contact layer prevents contact between the second anode active material layer and the solid electrolyte layer, wherein the contact layer comprises a second metal, and has a thickness less than a thickness of the first anode active material layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation Application of U.S. patent application Ser. No. 17/093,989, filed on Nov. 10, 2020, and issued as U.S. Pat. No. 11,605,812, which claims priority to and the benefit of Korean Patent Application No. 10-2020-0029166, filed on Mar. 9, 2020, in the Korean Intellectual Property Office, and all the benefits accruing therefrom under 35 U.S.C. § 119, the contents of which are incorporated herein by reference in their entirety.

BACKGROUND 1. Field

The present disclosure relates to an all-solid secondary battery and a method of preparing the all-solid secondary battery.

2. Description of the Related Art

Recently, in accordance with industrial demand, batteries having high energy density and high safety have been developed. For example, lithium-ion batteries have been put to practical use in the automotive field as well as in information-related equipment and communication equipment. In the field of automobiles, lithium-ion battery safety is particularly important.

Currently available lithium-ion batteries include an electrolytic solution including a flammable organic solvent, and thus when short-circuit occurs, there is a potential for overheating of the organic solvent and the occurrence of a fire. In this regard, an all-solid secondary battery including a solid electrolyte instead of an electrolytic solution has been proposed.

In an all-solid secondary battery, a flammable organic solvent is not used, and thus the potential for a fire to occur, or an explosion, even when short-circuit occurs, may be reduced. Therefore, an all-solid secondary battery may have greatly increased safety as compared to a lithium-ion battery using an electrolyte.

To increase the energy density of such an all-solid secondary battery, lithium may be used as an anode active material. For example, the capacity density (capacity per unit mass) of lithium is about 10 times greater than the capacity density of graphite (often used as an anode active material). Thus, when lithium is used as an anode active material, an all-solid secondary battery may be manufactured in the form of a thin film and may have an increased output.

However, there remains a need for improved all-solid secondary batteries in which a short circuit is less likely to occur.

SUMMARY

Provided is an all-solid secondary battery and a method of preparing the all-solid secondary battery, wherein the formation of cracks in a solid electrolyte may be prevented and an interfacial resistance between an anode layer and the solid electrolyte may be reduced in the all-solid secondary battery.

Additional aspects will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the presented embodiments of the disclosure.

According to an aspect of an embodiment, an all-solid secondary battery includes

a cathode,

an anode, and

a solid electrolyte layer disposed between the cathode and the anode,

wherein the anode includes

an anode current collector;

a first anode active material layer in contact with the anode current collector and including a first metal;

a second anode active material layer disposed between the first anode active material layer and the solid electrolyte layer and including a carbon-containing active material; and

a contact layer disposed between the second anode active material layer and the solid electrolyte layer, and disposed such that the contact layer prevents contact between the second anode active material layer and the solid electrolyte layer,

wherein the contact layer includes a second metal, and has a thickness less than that of the first anode active material layer.

The first metal may include lithium metal or a lithium alloy, and the second metal may include lithium metal or a lithium alloy.

The first metal and the second metal may be the same.

A thickness of the contact layer may be about 20% or less of the thickness of the first anode active material layer.

The thickness of the contact layer may be about 1 nanometer (nm) to about 1 micrometer (μm).

The thickness of the contact layer may be less than a thickness of the second anode active material layer.

During a charge/discharge cycle, a volume change rate of the first anode active material layer may be greater than a volume change rate of the contact layer.

During a charge/discharge cycle, a volume change rate of the second anode active material layer may be greater than a volume change rate of the contact layer.

A volume of the contact layer after charge may be about 1.5 times to about 20 times a volume of the contact layer after discharge.

A volume of the first anode active material layer after charge may be about 1.5 times to about 500 times a volume of the first anode active material layer after discharge.

A volume of the second anode active material layer after charge may be greater than a volume of the second anode active material layer after discharge and the volume of the second anode active material layer after charge may be about 2 times or less the volume of the second anode active material layer after discharge.

The solid electrolyte layer may include an oxide-containing solid electrolyte.

According to an aspect of another embodiment, a method of preparing an all-solid secondary battery includes

providing a cathode layer;

providing an anode layer;

providing a solid electrolyte layer;

attaching the anode layer to a surface of the solid electrolyte layer; and

attaching the cathode layer to another surface of the solid electrolyte layer,

wherein the providing of the anode layer includes:

-   -   disposing a first layer including lithium metal or a lithium         alloy on a first substrate;     -   disposing a second layer including a carbon-containing active         material on a second substrate;     -   disposing the first layer and the second layer to face each         other; and     -   pressing the first substrate and the second substrate such that         the first substrate and the second substrate move closer to each         other,

wherein in the process of pressing the first substrate and the second substrate, a third layer including lithium metal or a lithium alloy may be formed between the second substrate and the second layer, and the third layer has a thickness less than a thickness of the first layer.

The second layer may include a metal alloyable with lithium, and in the pressing of the first substrate and the second substrate the metal alloyable with lithium may form an alloy with lithium in the first layer and in the third layer.

The providing of the anode layer may further include removing the second substrate after the forming of the third layer.

A pressure applied in the pressing of the first substrate and the second substrate may be about 150 megapascals (MPa) to about 1,000 megapascals.

According to an aspect of another embodiment, a method of preparing an anode layer of an all-solid secondary battery includes

preparing a first layer including lithium metal or a lithium alloy disposed on a first substrate and a second layer including a carbon-based active material disposed on a second substrate;

disposing the first layer and the second layer to face each other; and

pressing the first substrate and the second substrate such that the first substrate and the second substrate to be close to each other,

wherein in the pressing of the first substrate and the second substrate, a third layer including lithium metal or a lithium alloy and having a thickness less than that of the first layer may be formed between the second substrate and the second layer.

The second layer may include a metal alloyable with lithium, and the metal may form alloy with lithium in the first layer and the third layer in the pressing of the first substrate and the second substrate.

In the preparing of the anode layer, the second substrate may be removed after the forming of the third layer.

A pressure applied in the pressing of the first substrate and the second substrate may be about 150 MPa or higher.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of certain embodiments of the disclosure will be more apparent from the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a cross-sectional view that illustrates an exemplary embodiment of an all-solid secondary battery;

FIG. 2 is a cross-sectional view that illustrates the anode layer in FIG. 1 ;

FIGS. 3 and 4 are cross-sectional views that illustrate charging and discharging of an exemplary embodiment of an anode layer of an all-solid secondary battery;

FIG. 5A is a scanning electron microscope (SEM) image of a cross-section of an exemplary embodiment of an anode layer after charge;

FIG. 5B is an SEM image of a cross-section of an exemplary embodiment of the anode layer after discharge;

FIG. 6A is an enlarged view of the SEM image of FIG. 5A, showing the periphery of the contact layer and the second anode active material layer;

FIG. 6B is an enlarged view the SEM image of FIG. 5B, showing the periphery of the contact layer and the second anode active material layer;

FIGS. 7A to 7D are views illustrating an exemplary embodiment of a method of preparing an anode layer;

FIG. 8 is an image of an exemplary embodiment of an anode layer;

FIG. 9 is a cross-sectional view of an anode layer of the Comparative Examples;

FIG. 10 is an image of the anode layer of the Comparative Examples;

FIG. 11 is a SEM image of a cross-section of an exemplary embodiment of an anode layer;

FIG. 12 is an enlarged view of a portion of FIG. 11 ;

FIG. 13 is an enlarged view of a portion of FIG. 12 ;

FIG. 14 is a Nyquist plot of imaginary impedance (Z″, ohm per square centimeter, ohm/cm²) versus real impedance (Z′, ohm/cm²) showing the impedance measurement results of Comparative Examples 1 and 2 and Example 1;

FIG. 15 is an enlarged view of a portion of FIG. 14 ; and

FIGS. 16 to 18 are each a graph of potential (volts versus Li/Li⁺) versus areal capacity (milliampere hours per square centimeter, mAh/cm²), showing the charge/discharge curves of all-solid secondary batteries including the anode layers prepared in accordance with Comparative Examples 1 and 2 and Example 1.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings. In this regard, the present embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, the embodiments are merely described below, by referring to the figures, to explain aspects.

It will be understood that when an element is referred to as being “on” another element, it can be directly on the other element or intervening elements may be present therebetween. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present.

It will be understood that, although the terms “first,” “second,” “third” etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, “a first element,” “component,” “region,” “layer” or “section” discussed below could be termed a second element, component, region, layer or section without departing from the teachings herein.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used herein, “a,” “an,” “the,” and “at least one” do not denote a limitation of quantity, and are intended to cover both the singular and plural, unless the context clearly indicates otherwise. For example, “an element” has the same meaning as “at least one element,” unless the context clearly indicates otherwise. “or” means “and/or.” As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Expressions such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. It will be further understood that the terms “comprises” and/or “comprising,” or “includes” and/or “including” when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.

“About” or “approximately” as used herein is inclusive of the stated value and means within an acceptable range of deviation for the particular value as determined by one of ordinary skill in the art, considering the measurement in question and the error associated with measurement of the particular quantity (i.e., the limitations of the measurement system). For example, “about” can mean within one or more standard deviations, or within ±30%, 20%, 10% or 5% of the stated value.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

Exemplary embodiments are described herein with reference to cross section illustrations that are schematic illustrations of idealized embodiments. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments described herein should not be construed as limited to the particular shapes of regions as illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, a region illustrated or described as flat may, typically, have rough and/or nonlinear features. Moreover, sharp angles that are illustrated may be rounded. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the precise shape of a region and are not intended to limit the scope of the present claims.

Hereinafter, an all-solid secondary battery according to an embodiment, an anode layer used in the all-solid secondary battery, and a method of preparing the all-solid secondary battery will be described in detail with reference to the accompanying drawings. Like reference numerals in the drawings denote like elements. Sizes of components in the drawings may be exaggerated for convenience of explanation. Hereinafter, one or more embodiments described below are merely illustrative, and various modifications are possible from these embodiments.

In an all-solid secondary battery including a solid electrolyte as an electrolyte and lithium as an anode active material, lithium metal may be irregularly deposited on a surface of the solid electrolyte during a charging process, and this may cause cracks in the solid electrolyte. The cracks of the solid electrolyte may result in a short-circuit of the all-solid secondary battery. The present disclosure provides an all-solid secondary battery including a thin contact layer including a metal between an anode active material layer and the solid electrolyte layer to minimize generation of cracks and the occurrence of a short-circuit.

FIG. 1 is a cross-sectional view illustrating an all-solid secondary battery 1 according to an embodiment, and FIG. 2 is a cross-sectional view illustrating an anode layer 20 in FIG. 1 . FIGS. 3 and 4 are views illustrating the effect of charge and discharge on the anode layer 20 of the all-solid secondary battery 1.

Referring to FIGS. 1 and 2 , the all-solid secondary battery 1 according to an embodiment is a secondary battery that includes a solid electrolyte as an electrolyte. For example, the all-solid secondary battery 1 may be an all-solid lithium ion secondary battery, in which lithium ions migrate between a cathode layer 20 and the anode layer 20.

The all-solid secondary battery 1 includes the cathode layer 10 (also referred to herein as the cathode), a solid electrolyte layer 30, and the anode layer 20 (also referred to herein as the anode).

Cathode Layer

The cathode layer 10 may include a cathode current collector 11 and a cathode active material layer 12.

For example, the cathode current collector 11 may be a plate or a foil including indium (In), copper (Cu), magnesium (Mg), stainless steel, titanium (Ti), iron (Fe), cobalt (Co), nickel (Ni), zinc (Zn), aluminum (Al), germanium (Ge), lithium (Li), an alloy thereof, or a combination thereof. The cathode current collector 11 may be omitted.

For example, the cathode active material layer 12 may include a cathode active material.

The cathode active material may be a cathode active material capable of reversibly absorbing and desorbing lithium ions. Examples of the cathode active material may include a lithium transition metal oxide, such as a lithium cobalt oxide (LCO), a lithium nickel oxide, a lithium nickel cobalt oxide, a lithium nickel cobalt aluminum oxide (NCA), a lithium nickel cobalt manganese oxide (NCM), a lithium manganate, a lithium iron phosphate, or a combination thereof; a nickel sulfide; a copper sulfide; a lithium sulfide; an iron oxide; a vanadium oxide; or a combination thereof, but embodiments are not limited thereto, and any material suitable for use a cathode active material may be used. The foregoing examples of the cathode active material may be used alone or in combination, for example, a mixture of at least two selected therefrom.

The lithium transition metal oxide may be, for example, a compound represented by one of the following formulae:

Li_(a)A_(1−b)B′_(b)D₂ (where 0.90≤a≤1 and 0≤b≤0.5); Li_(a)E_(1−b)B′_(b)O_(2−c)D_(c) (where 0.90≤a≤1, 0≤b≤0.5, and 0≤c≤0.05); LiE_(2−b)B′_(b)O_(4−c)D_(c) (where 0≤b≤0.5 and 0≤c≤0.05); Li_(a)Ni_(1−b−c)Co_(b)B′_(c)D_(α) (where 0.90≤a≤1, 0≤b≤0.5, 0≤c≤0.05, and 0<α≤2); Li_(a)Ni_(1−b−c)Co_(b)B′_(c)O_(2−α)F′_(α) (where 0.90≤a≤1, 0≤b≤0.5, 0≤c≤0.05, and 0<α<2); Li_(a)Ni_(1−b−c)Co_(b)B′_(c)O_(2−α)F′₂ (where 0.90≤a≤1, 0≤b≤0.5, 0≤c≤0.05, and 0<α<2); Li_(a)Ni_(1−b−c)Mn_(b)B′_(c)D_(α) (where 0.90≤a≤1, 0≤b≤0.5, 0≤c≤0.05, and 0<α≤2); Li_(a)Ni_(1−b−c)Mn_(b)B′_(c)O_(2−α)F′_(α) (where 0.90≤a≤1, 0≤b≤0.5, 0≤c≤0.05, and 0<α<2); Li_(a)Ni_(1−b−c)Mn_(b)B′_(c)O_(2−α)F′₂ (where 0.90≤a≤1, 0≤b≤0.5, 0≤c≤0.05, and 0<α<2); Li_(a)Ni_(b)E_(c)G_(d)O₂ (where 0.90≤a≤1, 0≤b≤0.9, 0≤c≤0.5, and 0.001≤d≤0.1); Li_(a)Ni_(b)Co_(c)Mn_(d)GeO₂ (where 0.90≤a≤1, 0≤b≤0.9, 0≤c≤0.5, 0≤d≤0.5, and 0.001≤e≤0.1); Li_(a)NiG_(b)O₂ (where 0.90≤a≤1 and 0.001≤b≤0.1); Li_(a)CoG_(b)O₂ (where 0.90≤a≤1 and 0.001≤b≤0.1); Li_(a)MnG_(b)O₂ (where 0.90≤a≤1 and 0.001≤b≤0.1); Li_(a)Mn₂G_(b)O₄ (where 0.90≤a≤1 and 0.001≤b≤0.1); QO₂, QS₂; LiQS₂; V₂O₅; LiV₂O₅; LiI′O₂; LiNiVO₄; Li_(3−f))J₂(PO₄)₃ (where 0≤f≤2); Li_((3−f))Fe₂(PO₄)₃ (where 0≤f≤2); and LiFePO₄. In the above compounds, A may be nickel (Ni), cobalt (Co), manganese (Mn), or a combination thereof; B′ may be aluminum (Al), nickel (Ni), cobalt (Co), manganese (Mn), chromium (Cr), iron (Fe), magnesium (Mg), strontium (Sr), vanadium (V), a rare earth element, or a combination thereof; D may be oxygen (O), fluorine (F), sulfur (S), phosphorus (P), or a combination thereof; E may be cobalt (Co), manganese (Mn), or a combination thereof; F′ may be fluorine (F), sulfur (S), phosphorus (P), or a combination thereof; G may be (Al), chromium (Cr), manganese (Mn), iron (Fe), magnesium (Mg), lanthanum (La), cerium (Ce), strontium (Sr), vanadium (V), or a combination thereof; Q may be titanium (Ti), molybdenum (Mo), manganese (Mn), or a combination thereof; I′ may be chromium (Cr), vanadium (V), iron (Fe), scandium (Sc), yttrium (Y), or a combination thereof; and J may be vanadium (V), chromium (Cr), manganese (Mn), cobalt (Co), nickel (Ni), copper (Cu), or a combination thereof.

The above compounds may have a surface coating layer (hereinafter, also referred to as “coating layer”). Alternatively, a mixture of a compound without a coating layer and a compound having a coating layer may be used, where the compounds are selected from the compounds listed above. In some embodiments, the coating layer may include at least one of an oxide, a hydroxide, an oxyhydroxide, an oxycarbonate, or a hydroxycarbonate of the coating element. In some embodiments, the compounds for the coating layer may be amorphous or crystalline. In some embodiments, the coating element for the coating layer may be magnesium (Mg), aluminum (Al), cobalt (Co), potassium (K), sodium (Na), calcium (Ca), silicon (Si), titanium (Ti), vanadium (V), tin (Sn), germanium (Ge), gallium (Ga), boron (B), arsenic (As), zirconium (Zr), or a combination thereof. In some embodiments, the coating layer may be formed using any method that does not adversely affect the physical properties of the cathode active material when a compound of the coating element is used. For example, the coating layer may be formed using a spray coating method or a dipping method. The coating methods are well understood by one of ordinary skill in the art, and thus a detailed description thereof is omitted herein.

The cathode active material may include, for example, a lithium transition metal oxide including a lithium salt of a transition metal oxide that has a layered rock-salt type structure. For example, the term “layered rock-salt type structure” refers to a structure in which an oxygen atom layer and a metal atom layer are alternately and regularly arranged in a <111> direction in a cubic rock-salt type structure, and where each of the atom layers forms a two-dimensional flat plane. The term “cubic rock-salt type structure” refers to a sodium chloride (NaCl) type structure, which is one of the known crystalline structures, in which face-centered cubic (fcc) lattices respectively formed of anions and cations are shifted by only a half of the ridge of each unit lattice. Examples of the lithium transition metal oxide having a layered rock-salt type structure may include a ternary lithium transition metal oxide such as LiNi_(x)Co_(y)Al_(z)O₂ (NCA) or LiNi_(x)Co_(y)Mn_(z)O₂ (NCM) (where 0<x<1, 0<y<1, 0<z<1, and x+y+z=1). When the cathode active material includes a ternary transition metal oxide having the layered rock-salt type structure, an energy density and thermal stability of the all-solid secondary battery 1 may improve.

The cathode active material may include a coating layer as described above. The coating layer is not limited and may be any suitable material for use as a coating layer of a cathode active material of an all-solid secondary battery. The coating layer may be, for example, Li₂O—ZrO₂.

When the cathode active material includes nickel (Ni) as a ternary lithium transition metal oxide such as NCA or NCM, a capacity density of the all-solid secondary battery 1 increases, and thus metal elution from the cathode active material in a charged state may be reduced. As a result, the all-solid secondary battery 1 according to an embodiment may have improved cycle characteristics in a charged state.

The cathode active material may have, for example, a particle shape such as a true spherical shape, an elliptical shape, or a semi-spherical shape. A particle diameter of the cathode active material is not particularly limited, but may be in a range suitable for a cathode active material of an all-solid secondary battery. An amount of the cathode active material in the cathode layer 10 is not particularly limited and may be in a range suitable for a cathode layer of an all-solid secondary battery.

Additives such as a conducting agent, a binder, a filler, a dispersant, and an ion conducting agent may be included in the cathode layer 10, in addition to the cathode active material. Examples of the conducting agent may include graphite, carbon black, acetylene black, ketjen black, carbon fiber, a metal powder, or a combination thereof. Examples of the binder may include styrene butadiene rubber (SBR), polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, or a combination thereof. The coating agent, the dispersant, and the ion conducting agent may be appropriately added to the cathode layer 10, and may be any material that suitable for use in an electrode of an all-solid secondary battery.

The cathode layer 10 may further include a solid electrolyte. The solid electrolyte in the cathode layer 10 may be similar to or different from the solid electrolyte in the solid electrolyte layer 30. Details of the solid electrolyte in the cathode layer are the same those described with reference to the solid electrolyte layer 30.

The solid electrolyte in the cathode layer 10 may be, for example, a sulfide-based (e.g., sulfide-containing) solid electrolyte. The sulfide-based solid electrolyte may also be used as a sulfide-based solid electrolyte in the solid electrolyte layer 30

In some embodiments, the cathode layer 10 may be, for example, impregnated in a liquid electrolyte. The liquid electrolyte may include a lithium salt and at least one of an ionic liquid or a polymer ionic liquid. The liquid electrolyte may be non-volatile. The ionic liquid refers to a salt in a liquid state at room temperature or a room temperature molten salt that has a melting point of room temperature or lower and is only formed of ions. The ionic liquid may be a compound including: a) a cation selected from an ammonium-based cation, a pyrrolidinium-based cation, a pyridinium-based cation, a pyrimidinium-based cation, an imidazolium-based cation, a piperidinium-based cation, a pyrazolium-based cation, an oxazolium-based cation, a pyridazinium-based cation, a phosphonium-based cation, a sulfonium-based cation, a triazolium-based cation, or a combination thereof; and b) an anion selected from BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻, SbF₆ ⁻, AlCl₄ ⁻, HSO₄ ⁻, ClO₄ ⁻, CH₃SO₃ ⁻, CF₃CO₂ ⁻, Cl⁻, Br⁻, I⁻, BF₄ ⁻, SO₄ ⁻, CF₃SO₃ ⁻, (FSO₂)₂N⁻, (C₂F₅SO₂)₂N⁻, (C₂F₅SO₂)(CF₃SO₂)N⁻, and (CF₃SO₂)₂N⁻. The ionic liquid may be, for example, at least one selected from the group consisting of N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide, N-butyl-N-methylpyrrolidinium bis(3-trifluoromethylsulfonyl)imide, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, or a combination thereof.

The polymer ionic liquid may have a repeating unit including: a) a cation selected from an ammonium-based cation, a pyrrolidinium-based cation, a pyridinium-based cation, a pyrimidinium-based cation, an imidazolium-based cation, a piperidinium-based cation, a pyrazolium-based cation, an oxazolium-based cation, a pyridazinium-based cation, a phosphonium-based cation, a sulfonium-based cation, a triazolium-based cation, or a combination thereof; and b) an anion selected from BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻, SbF₆ ⁻, AlCl₄ ⁻, HSO₄ ⁻, ClO₄ ⁻, CH₃SO₃ ⁻, CF₃CO₂ ⁻, (CF₃SO₂)₂N⁻, (FSO₂)₂N⁻, Cl⁻, Br⁻, I⁻, SO₄ ⁻, CF₃SO₃ ⁻, (C₂F₅SO₂)₂N⁻, (C₂F₅SO₂)(CF₃SO₂)N⁻, NO₃ ⁻, Al₂Cl₇ ⁻, (CF₃SO₂)₃C⁻, (CF₃)₂PF₄ ⁻, (CF₃)₃PF₃ ⁻, (CF₃)4PF₂ ⁻, (CF₃)₅PF⁻, (CF₃)₆P⁻, SF₅CF₂SO₃ ⁻, SF₅CHFCF₂SO₃ ⁻, CF₃CF₂(CF₃)₂CO⁻, CF₃SO₂)₂CH⁻, (SF₅)₃C⁻, (O(CF₃)₂C₂(CF₃)₂O)₂PO⁻, or a combination thereof. The lithium salt is not limited and may be any suitable lithium salt. The lithium salt may be, for example, LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiClO₄, LiCF₃SO₃, Li(CF₃SO₂)₂N, Li(FSO₂)₂N, LiC₄F₉SO₃, LiAlO₂, LiAlCl₄, LiN(C_(x)F_(2x−1)SO₂)(C_(y)F_(2y+1)SO₂) (where x and y are each a natural number), LiCl, LiI, or a combination thereof. A concentration of the lithium salt in the liquid electrolyte may be, for example, in a range of about 0.1 molar (M) to about 5 M, or about 0.05 M to about 4.0 M, or about 0.1 M to about 3.0 M, or about 0.5 M to about 2.0 M, or about 0.5 M to about 1.5 M. An amount of the liquid electrolyte in the cathode layer 10 may be, for example, in a range of 0 part to about 100 parts by weight, 0 part to about 50 parts by weight, 0 part to about 30 parts by weight, 0 part to about 20 parts by weight, 0 part to about 10 parts by weight, or 0 part to about 50 parts by weight, based on 100 parts by weight of the cathode active material layer 12 not including the liquid electrolyte.

Solid Electrolyte Layer

The solid electrolyte layer 30 may be disposed between the cathode layer 10 and the anode layer 20. The solid electrolyte layer 30 includes a solid electrolyte.

The solid electrolyte may be, for example, an oxide-based (e.g. oxide-containing) inorganic solid electrolyte. The oxide-based solid electrolyte may include Li_(1+x+y)Al_(x)Ti_(2−x)Si_(y)P_(3−y)O₁₂ (where 0<x<2 and 0≤y<3), BaTiO₃, Pb(Zr,Ti)O₃ (PZT), Pb_(1−x)La_(x)Zr_(1−y) Ti_(y)O₃ (PLZT) (where 0≤x<1 and 0≤y<1), PB(Mg₃Nb_(2/3))O₃—PbTiO₃ (PMN-PT), HfO₂, SrTiO₃, SnO₂, CeO₂, Na₂O, MgO, NiO, CaO, BaO, ZnO, ZrO₂, Y₂O₃, Al₂O₃, TiO₂, SiO₂, Li₃PO₄, Li_(x)Ti_(y)(PO₄)₃ (where 0<x<2 and 0<y<3), Li_(x)Al_(y)Ti_(z)(PO₄)₃ (where 0<x<2, 0<y<1, and 0<z<3), Li_(1+x+y)(Al, Ga)_(x)(Ti, Ge)_(2−x)Si_(y)P_(3−y)O₁₂ (where 0≤x≤1 and 0≤y≤1), Li_(x)La_(y)TiO₃ (where 0<x<2 and 0<y<3), Li₂O, LiOH, Li₂CO₃, LiAlO₂, Li₂O—Al₂O₃—SiO₂—P₂O₅—TiO₂—GeO₂, Li_(3+x)La₃M₂O₁₂ (where M is Te, Nb, or Zr, and x is an integer of 1 to 10), or a combination thereof. The solid electrolyte may be prepared using a sintering method.

The oxide-based solid electrolyte may be, for example, a garnet-type solid electrolyte selected from Li₇La₃Zr₂O₁₂ (LLZO) and Li_(3+x)La₃Zr_(2−a)M_(a)O₁₂ (M-doped LLZO, where M is Ga, W, Nb, Ta, or Al, and x is an integer of 1 to 10).

In some embodiments, the solid electrolyte may be, for example, a sulfide-based solid electrolyte. The sulfide-based solid electrolyte may include, for example, Li₂S—P₂S₅, Li₂S—P₂S₅—LiX (where X is a halogen), Li₂S—P₂S₅—Li₂O, Li₂S—P₂S₅—Li₂O—LiI, Li₂S—SiS₂, Li₂S—SiS₂—LiI, Li₂S—SiS₂—LiBr, Li₂S—SiS₂—LiCl, Li₂S—SiS₂—B₂S₃—LiI, Li₂S—SiS₂—P₂S₅—LiI, Li₂S—B₂S₃, Li₂S—P₂S₅—Z_(m)S_(n) (where m and n are positive integers, and Z is Ge, Zn, or Ga), Li₂S—GeS₂, Li₂S—SiS₂—Li₃PO₄, Li₂S—SiS₂—Li_(p)MO_(q) (where p and q are positive integers, and M is P, Si, Ge, B, Al, Ga, In, or a combination thereof), Li_(7−x)PS_(6−x)Cl_(x) (where 0≤x≤2), Li_(7−x)PS_(6−x)Br_(x) (where 0≤x≤2), and Li_(7−x)PS_(6−x)I_(x) (where 0≤x≤2). The sulfide-based solid electrolyte material is prepared by melting and quenching the starting materials (e.g., Li₂S or P₂S₅), or by mechanical milling of the starting materials. Subsequently, the resultant may be heat-treated. The sulfide-based solid electrolyte may be amorphous, or crystalline, or a mixed form thereof.

The sulfide-based solid electrolyte may include sulfur (S), phosphorus (P), and lithium (Li), as component elements. For example, the sulfide-based solid electrolyte may be a material including Li₂S—P₂S₅. When the material including Li₂S—P₂S₅ is used as a sulfide-based solid electrolyte, a mixing molar ratio of Li₂S and P₂S₅ (Li₂S:P₂S₅) may be, for example, in a range of about 50:50 to about 90:10.

The sulfide-based solid electrolyte may be an argyrodite-type compound including Li_(7−x)PS_(6−x)Cl_(x) (where 0≤x≤2), Li_(7−x)PS_(6−x)Br_(x) (where 0≤x≤2), Li_(7−x)PS_(6−x)I_(x) (where 0≤x≤2), or a combination thereof. In particular, the sulfide-based solid electrolyte may be an argyrodite-type compound including Li₆PS₅Cl, Li₆PS₅Br, Li₆PS₅I, or a combination thereof.

For example, the solid electrolyte layer 30 may further include a binder. Examples of the binder in the solid electrolyte layer 30 include styrene butadiene rubber (SBR), polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, or a combination thereof, but is not limited thereto, and any material suitable for use as a binder may be used. The binder of the solid electrolyte 30 may be the same as or different from a binder of the cathode layer and/or the anode layer.

Anode Layer

Referring to FIGS. 1 and 2 , the anode layer 20 includes an anode current collector 21, an anode active material layer 22, and a contact layer 23.

During a process of charging the all-solid secondary battery 1, a volume of the anode layer 20 may increase as shown in FIG. 3 . During a process of discharging the all-solid secondary battery 1, a volume of the anode layer 20 may decrease as shown in FIG. 4 .

For example, the anode current collector 21 may be formed of a material that does not react with lithium, that is, a material neither forming an alloy with lithium nor a compound with lithium. Examples of material forming the anode current collector 21 may include copper (Cu), stainless steel, titanium (Ti), iron (Fe), cobalt (Co), nickel (Ni), or a combination thereof, but embodiments are not limited thereto, and any material available suitable for use as an electrode (anode) current collector in the art may be used. The anode current collector 21 may include any one metal alone, an alloy of at least two different metals, or a coating material. The anode current collector 21 may be, for example, in the form of a plate or a foil.

The anode active material layer 22 may include a first anode active material layer 221 and a second anode active material layer 222.

The first anode active material layer 221 may be disposed on the anode current collector 21 and may include a first metal. The first anode active material layer 221 may thus be a first metal layer. The first metal may include lithium metal or a lithium alloy. Accordingly, since the first anode active material layer 221 is a metal layer including lithium or a lithium alloy, for example, the first anode active material layer 221 may function as a lithium reservoir.

The lithium metal refers to metallic lithium and thus consists of lithium (Li), and does not include a metal alloyable with lithium. The lithium alloy includes lithium and a metal alloyable with lithium. Examples of the lithium alloy may include a Li—Ag alloy, a Li—Au alloy, a Li—Al alloy, a Li—Sn alloy, a Li—In alloy, a Li—Zn alloy, a Li—Ge alloy, a Li—Si alloy, or a combination thereof, but embodiments are not limited thereto, and any material suitable as a lithium alloy may be used. The first anode active material layer 221 may include one of the lithium alloys or lithium metal, or may include a combination of various alloys selected therefrom.

A thickness t1 of the first anode active material layer 221 may be about 10 μm or greater. For example, the thickness t1 of the first anode active material layer 221 may be in a range of about 10 μm to about 1,000 μm, about 10 μm to about 500 μm, about 10 μm to about 200 μm, about 10 μm to about 150 μm, about 10 μm to about 100 μm, or about 10 μm to about 50 μm. When the thickness t1 of the first anode active material layer 221 is too thin, the first anode active material layer 221 may not function as a lithium reservoir. When the thickness t1 of the first anode active material layer 221 is too thick, a weight and a volume of the all-solid secondary battery 1 increases, and cycle characteristics of the all-solid secondary battery 1 may be deteriorated.

The first anode active material layer 221 may be disposed between the anode current collector 21 and the second anode active material layer 222. When the all-solid secondary battery 1 is charged, lithium is deposited in the first anode active material layer 221, and a volume or a thickness of the first anode active material layer 221 may increase due to the deposited lithium.

A volume of the first anode active material layer 221 after charge may be about 150% to about 5,000% of a volume of the first anode active material layer 221 after discharge. A thickness of the first anode active material layer 221 after charge may be about 150 to about 5,000% of a thickness of the first anode active material layer 221 after discharge.

In a charging/discharging process (e.g., a charge/discharge cycle), a volume change rate of the first anode active material layer 221 may be greater than a volume change rate of the contact layer 23. In the charging/discharging process, a volume change rate of the first anode active material layer 221 may be greater than a volume change rate of the second anode active material layer 222. In the charging/discharging process, a thickness change rate of the first anode active material layer 221 may be greater than a thickness change rate of the contact layer 23. In the charging/discharging process, a thickness change rate of the first anode active material layer 221 may be greater than a thickness change rate of the second anode active material layer 222.

The second anode active material layer 222 may be disposed between the first anode active material layer 221 and the solid electrolyte layer 30 and may include a carbon-based (e.g., carbon-containing) active material.

The carbon-based active material may include amorphous carbon. Examples of the amorphous carbon may include carbon black (CB), acetylene black (AB), furnace black (FB), furnace black (FB), ketjen black (KB), graphene, carbon nanotubes, carbon nanofibers, or a combination thereof, but is not limited thereto, and any material classified as an amorphous carbon may be used.

The second anode active material layer 222 may include a metal or a metalloid as an anode active material. As used herein, “metalloid” means B, Si, Ge, As, Sb, Te, or a combination thereof.

The metal or metalloid anode active material may include indium (In), silicon (Si), gallium (Ga), tin (Sn), aluminum (Al), titanium (Ti), zirconium (Zr), niobium (Nb), germanium (Ge), antimony (Sb), bismuth (Bi), gold (Au), platinum (Pt), palladium (Pd), magnesium (Mg), silver (Ag), zinc (Zn), or a combination thereof, but is not limited thereto, and any metal anode active material or metalloid anode active material capable of forming an alloy or a compound with lithium may be used.

The second anode active material layer 222 may include an anode active material including a carbon-based active material, a metal or a metalloid anode active material, or a combination of the carbon-based active material and the metal or metalloid active material. For example, the second anode active material layer 222 may include only amorphous carbon, or the second anode active material layer may include indium (In), silicon (Si), gallium (Ga), tin (Sn), aluminum (Al), titanium (Ti), zirconium (Zr), niobium (Nb), germanium (Ge), antimony (Sb), bismuth (Bi), gold (Au), platinum (Pt), palladium (Pd), magnesium (Mg), silver (Ag), zinc (Zn), or a combination thereof. In some embodiments, the second anode active material 222 may include a composite of amorphous carbon and a metal or a metalloid including indium (In), silicon (Si), gallium (Ga), tin (Sn), aluminum (Al), titanium (Ti), zirconium (Zr), niobium (Nb), germanium (Ge), antimony (Sb), bismuth (Bi), gold (Au), platinum (Pt), palladium (Pd), magnesium (Mg), silver (Ag), zinc (Zn), or a combination thereof. A composite ratio of a composite of amorphous carbon and a metal (e.g., silver) or metalloid is a weight ratio, which may be, for example, in a range of about 10:1 to about 1:2, about 5:1 to about 1:1, or about 4:1 to about 2:1, but embodiments are not limited thereto, and the composite ratio may be determined by the person of skill in the art according to the desired characteristics of the all-solid secondary battery 1. When the second anode active material layer 222 has this composition, cycle characteristics of the all-solid secondary battery 1 may improve.

The anode active material in the second anode active material layer 222 may include, for example, a mixture of first particles of an amorphous carbon and second particles of a metal or a metalloid. The mixture may comprise, consist of, or consist essentially of a dispersion of the first particle and the second particle. Alternatively, the mixture may further include a binder and the first particle and the second particles are physically bound together by a binder. For example, the metal or metalloid may include indium (In), silicon (Si), gallium (Ga), tin (Sn), aluminum (Al), titanium (Ti), zirconium (Zr), niobium (Nb), germanium (Ge), antimony (Sb), bismuth (Bi), gold (Au), platinum (Pt), palladium (Pd), magnesium (Mg), silver (Ag), zinc (Zn), or a combination thereof. In some embodiments, the metal or metalloid may be a semiconductor. An amount of the second particles may be in a range of about 8 weight % (wt %) to about 60 wt %, about 10 wt % to about 50 wt %, about 15 wt % to about 40 wt %, or about 20 wt % to about 30 wt %, based on the total weight of the mixture. When the amount of the second particles is within these ranges, for example, cycle characteristics of the all-solid secondary battery 1 may improve.

A thickness t2 of the second anode active material layer 222 may be, for example, in a range of about 10 nm to about 10 μm, about 100 nm to about 10 μm, about 200 nm to about 10 μm, about 300 nm to about 10 μm, about 400 nm to about 10 μm, about 500 nm to about 10 μm, about 1 μm to about 10 μm, about 1 μm to about 8 μm, about 2 μm to about 7 μm, or about 3 μm to about 7 μm. When the thickness of the second anode active material layer 222 is within these ranges, a short-circuit in the all-solid secondary battery 1 may be suppressed, and cycle characteristics of the all-solid secondary battery 1 may improve.

The thickness t2 of the second anode active material layer 222 may be less than the thickness t1 of the first anode active material layer 221. The thickness t2 of the second anode active material layer 222 may be less than ½ of the thickness t1 of the first anode active material layer 221. The thickness t2 of the second anode active material layer 222 may be less than 20% of the thickness t1 of the first anode active material layer 221.

When the second anode active material layer 222 includes a carbon-based active material, the volume of the second anode active material layer 222 may change according to a volume change of the first anode active material layer 221. For example, when the first anode active material layer 221 expands during a charging process, the second anode active material layer 222 may absorb and alleviate the volume expansion of the first anode active material.

The second anode active material layer 222 includes a carbon-based active material and thus may include a void (pore) therein. The second anode active material layer 222 after discharge may include a void generated therein. During a charging process, lithium fills the void of the second anode active material layer 222, and in this regard, the volume expansion of the first anode active material layer 221 may be alleviated. As the volume expansion of the first anode active material layer 221 is alleviated, a pressure applied by the anode layer 20 on the solid electrolyte layer 30 is reduced, and thus a short-circuit of the solid electrolyte layer 30 may be delayed.

A volume of the second anode active material layer 222 after charging may be greater than a volume of the second anode active material layer 222 after discharging. The volume of the second anode active material layer 222 after charging may be about 2 times or less the volume of the second anode active material layer 222 after discharging. A thickness t21 of the second anode active material layer 222 after charging may be greater than a thickness t22 of the second anode active material layer 222 after discharging. The thickness t21 of the second anode active material layer 222 after charging may be about 2 times or less than the thickness t22 of the second anode active material layer 222 after discharging. In other words, the volume and/or thickness of the second anode active material layer after charge does not increase more than 2 fold relative to the volume and/or thickness of the second anode active material layer during discharge.

When the second anode active material layer 222 includes a carbon-based active material as described above, volume expansion of the first anode active material layer 221 may be alleviated, whereas an interface adhesive strength with the solid electrolyte layer may be deteriorated. Accordingly, when the second anode active material layer 222 is disposed such that the second anode active material layer 222 directly contacts the solid electrolyte layer 30, an interfacial resistance between the anode layer 20 and the solid electrolyte layer 30 may increase.

In this regard, the anode layer 20 of the all-solid secondary battery 1 according to an embodiment includes the contact layer 23 disposed between the second anode active material layer 222 and the solid electrolyte layer 30.

At least a portion of the contact layer 23 is disposed between the second anode active material layer 222 and the solid electrolyte layer 30 and directly contacts the solid electrolyte layer 30. As the contact layer 23 directly contacts the solid electrolyte layer 30, the second anode active material layer 222 may be prevented from directly contacting the solid electrolyte layer 30. Thus the contact layer is between the second anode active material layer and the solid electrolyte layer, and is disposed such that the contact layer prevents contact between the second anode active material layer and the solid electrolyte layer. Accordingly, the contact layer 23 may improve an interface adhesive strength between the anode layer 20 and the solid electrolyte layer 30.

An interfacial resistance between the contact layer 23 and the solid electrolyte layer 30 may be a prescribed level or less. For example, an interfacial resistance between the contact layer 23 and the solid electrolyte layer 30 may be about 500 ohm·cm² or less, or about 400 ohm·cm² or less, or about 300 ohm·cm² or less. For example, an interfacial resistance between the contact layer 23 and the solid electrolyte layer 30 may be about 200 ohm·cm² or less.

An interfacial resistance between the contact layer 23 and the solid electrolyte layer 30 may be lower than an interfacial resistance between the second anode active material layer 222 and the solid electrolyte layer 30. For example, an interfacial resistance between the contact layer 23 and the solid electrolyte layer 30 may be less than 10% ( 1/10) of an interfacial resistance between the second anode active material layer 222 and the solid electrolyte layer 30. For example, when an interfacial resistance between the second anode active material layer 222 and the solid electrolyte layer 30 directly contacting each other is greater than about 2,000 ohm·cm², an interfacial resistance between the contact layer 23 and the solid electrolyte layer 30 directly contacting each other may be about 200 ohm·cm² or less.

The contact layer 23 may include a second metal including lithium metal or a lithium alloy. The contact layer thus may be a second metal layer. The second metal may include lithium metal or a lithium alloy.

The lithium metal refers to metallic lithium and thus consists of lithium, and does not include a metal alloyable with lithium.

The lithium alloy includes lithium and a metal alloyable with lithium. Examples of the lithium alloy may include a Li—Al alloy, a Li—Sn alloy, a Li—In alloy, a Li—Ag alloy, a Li—Au alloy, a Li—Zn alloy, a Li—Ge alloy, a Li—Si alloy, or a combination thereof, but embodiments are not limited thereto, and any material suitable as a lithium alloy may be used.

The contact layer 23 may include one of the lithium alloy or lithium metal, or may include a combination of various alloys selected therefrom.

The second metal may be the same material as the first metal. However, a material of the second metal is not limited thereto, and may be different from a material of the first metal.

For example, the contact layer 23 may not include a carbon-based material. For example, the contact layer 23 may not include a carbon-based material such as a carbon-based active material, e.g., graphite or carbon black, or a carbon-based conducting material, e.g., carbon nanofibers. The contact layer 23 may not include an organic material such as a binder. The contact layer 23 may include, for example, a metal layer formed of a metal, a metalloid, an alloy thereof, or a combination thereof. Since the contact layer 23 is a metal layer and does not include a carbon-based material, a side reaction between a carbon-based material and/or an organic material in a charge/discharge process (cycle) may be prevented.

Also, since the contact layer 23 includes the second metal layer and does not include a carbon-based material, the contact layer 23 may form an interface having an excellent adhesive strength to the solid electrolyte layer 30 compared to that of the second anode active material layer 222 including a carbon-based active material.

Since the contact layer 23 includes the second metal, the contact layer 23 induces fast dispersion of the lithium ions input through the solid electrolyte layer 30 in the charging process. Accordingly, a surface of the solid electrolyte layer 30 is irregular, and thus even when the lithium ions are input to solid electrolyte layer while being locally focused by the contact layer, the lithium ions may be evenly dispersed throughout the whole anode layer 20 by using a fast diffusion phenomenon via the contact layer 23.

In the charging process, the contact layer 23 may be configured such that an amount of the metal, e.g., lithium metal or a lithium alloy, deposited during formation of the contact layer is less than an amount of the first metal in the first anode active material layer 221.

A thickness t3 of the contact layer 23 may be a predetermined thickness or less. For example, the thickness t3 of the contact layer 23 may be about 1 μm or less. For example, the thickness t3 of the contact layer 23 may be about 0.5 μm or less. For example, the thickness t3 of the contact layer 23 may be about 0.1 μm or less. However, when the thickness t3 of the contact layer 23 is too thin, the original purpose for the second anode active material layer 222 to directly contact the solid electrolyte layer 30 may not be achieved, and thus the thickness t3 of the contact layer 23 may be about 1 nm or greater. The thickness of the contact layer may thus be about 1 nm to about 1 μm, or about 1 nm to about 500 nm, or about 1 nm to about 100 nm, or about 1 nm to about 50 nm. The thickness of the contact layer may be uniform or may not be uniform. Here, when the thickness t3 of the contact layer 23 is not uniform, the thickness t3 of the contact layer 23 is defined as an average thickness of the contact layer 23.

The thickness t3 of the contact layer 23 is less than the thickness t1 of the first anode active material layer 221. For example, the thickness t3 of the contact layer 23 after discharge may be less than the thickness t12 of the first anode active material layer 221 after discharge. When the thickness t12 of the first anode active material layer 221 after discharge is greater than about 10 μm, a thickness t32 of the contact layer 23 after discharging may be about 1 μm or less.

The thickness t3 of the contact layer 23 may be about 20% (⅕) or less of the thickness t1 of the first anode active material layer 221. The thickness t3 of the contact layer 23 may be about 10% ( 1/10) or less of the thickness t1 of the first anode active material layer 221. The thickness t3 of the contact layer 23 may be about 5% ( 1/20) or less of the thickness t1 of the first anode active material layer 221.

The thickness t32 of the contact layer 23 after discharge may be about 20% (⅕) or less of the thickness t12 of the first anode active material layer 221 after discharge. The thickness t32 of the contact layer 23 after discharge may be about 10% ( 1/10) or less of the thickness t12 of the first anode active material layer 221 after discharge. The thickness t32 of the contact layer 23 after discharging may be about 5% ( 1/20) or less of the thickness t12 of the first anode active material layer 221 after discharging.

The thickness t31 of the contact layer 23 after charge may be about 10% ( 1/10) or less of the thickness t11 of the first anode active material layer 221 after charge. The thickness t31 of the contact layer 23 after charge may be about 5% ( 1/20) or less of the thickness t11 of the first anode active material layer 221 after charge. The thickness t31 of the contact layer 23 after charge may be about 2.5% ( 1/40) or less of the thickness t11 of the first anode active material layer 221 after charge.

Accordingly, by designing the thickness t3 of the contact layer 23 to be a predetermined thickness (or less) and the thickness t1 of the first anode active material layer 221 to be greater than the thickness of the contact layer 23, an amount of the metal deposited as the contact layer 23 during charge of the all-solid secondary battery 1 may be decreased, and the metal may be induced to be deposited in the first anode active material layer 221.

When the thickness t3 of the contact layer 23 is greater than the predetermined thickness, an amount of the lithium metal locally deposited as the contact layer 23 during the charging process may increase. This may generate cracks in the solid electrolyte layer 30.

In particular, when the solid electrolyte layer 30 includes an oxide-based solid electrolyte, which has a greater hardness than a sulfide-based solid electrolyte, cracks may be generated in the solid electrolyte layer 30 due to the localized deposition of the lithium metal in the contact layer 23, and the lithium metal may penetrate the solid electrolyte layer 30 through the cracks. The penetration of the lithium metal into the solid electrolyte layer may cause a short circuit which may deteriorate stability of the all-solid secondary battery 1.

Also, due to the lithium metal locally deposited as the contact layer 23, in the repeated charge/discharge process, a void may be formed between the contact layer 23 and the solid electrolyte layer 30, and a contact area between the contact layer 23 and the solid electrolyte layer 30 may be reduced, which may lead to overvoltage of the all-solid secondary battery 1.

However, in the all-solid secondary battery 1 according to an embodiment, the metal is also deposited in the first anode active material layer 221, and thus an amount of the lithium metal deposited in the contact layer 23 may be reduced. In this regard, the short circuiting and overcharge of the all-solid secondary battery 1 may be prevented.

When an amount of lithium deposited in the contact layer 23 in the charging process is reduced, a volume change rate of the contact layer 23 in the charge/discharge process may be small.

For example, a volume of the contact layer 23 after charge may be about 150% or less of a volume of the contact layer 23 after discharge. A volume of the contact layer 23 after charge may be about 140% or less of a volume of the contact layer 23 after discharge. A volume of the contact layer 23 after charge may be about 130% or less of a volume of the contact layer 23 after discharge.

For example, in the charge/discharge process, a volume change rate of the contact layer 23 may be less than a volume change rate of the first anode active material layer 221. In the charge/discharge process, a volume change rate of the contact layer 23 may be about 70% or less of a volume change rate of the first anode active material layer 221. In the charge/discharge process, a volume change rate of the contact layer 23 may be about 60% or less of a volume change rate of the first anode active material layer 221. A volume change rate of the contact layer may be about 5% to about 70% of a volume change rate of the first anode active material layer, or a volume change rate of the contact layer may be about 10% to about 60% of a volume change rate of the first anode active material layer, or a volume change rate of the contact layer may be about 10% to about 50% of a volume change rate of the first anode active material layer. The second metal of the contact layer 23 may be the same as the first metal of the first anode active material layer 221. For example, the second metal and the first metal may both be lithium metal. For example, the second metal and the first metal may both be a lithium alloy, and the metal forming an alloy with lithium may be the same.

However, the second metal is not necessarily the same as the first metal layer, and the material may vary according to the preparation method or desired used.

FIGS. 5A and 5B are, respectively, a cross-section scanning electron microscope (SEM) image after charge and a cross-section SEM image after discharge of an exemplary embodiment of the anode layer 20. FIGS. 6A and 6B are, respectively, an enlarged cross-section SEM image of the periphery of the contact layer 23 and the second anode active material layer 222 in FIG. 5A and an enlarged cross-section SEM image of the periphery of the contact layer 23 and the second anode active material layer 222 in FIG. 5B.

Referring to FIGS. 5A and 5B, a thickness t11 of the first anode active material layer 221 after charge is in a range of about 32 μm to about 34 μm, and a thickness t12 of the first anode active material layer 221 after discharge is in a range of about 17 μm to about 18 μm.

On the other hand, referring to FIGS. 6A and 6B, a thickness t21 of the second anode active material layer 222 after charge is in a range of about 5 μm to about 6 μm, and a thickness t22 of the second anode active material layer 222 after discharge is also in a range of about 5 μm to about 6 μm. Also, a thickness t31 of the contact layer 23 after charge is in a range of about 0.5 μm to about 1.5 μm, and a thickness t32 of the contact layer 23 after discharge is also in a range of about 0.5 μm to about 1.5 μm.

In this regard, it may be confirmed that for the anode layer 20 according to an embodiment, a thickness change rate of the contact layer 23 and a thickness change rate of the second anode active material layer 222 are each less than a thickness change rate of the first anode active material layer 221.

A method of preparing the all-solid secondary battery 1 according to an embodiment includes providing an anode layer 20; disposing the anode layer 20 on a surface of a solid electrolyte layer 30; and disposing a cathode layer 10 on another surface of the solid electrolyte layer 30.

(Preparation of Anode Layer 20)

FIGS. 7A to 7D are illustrations of an exemplary embodiment of a method of preparing the anode layer 20.

Referring to FIG. 7A, a first layer 321 is formed on a first substrate 100.

The first layer 321 may include a metal layer. The metal layer may include lithium metal, a lithium alloy, or a combination thereof.

The lithium alloy may include a Li—Ag alloy, a Li—Au alloy, a Li—Al alloy, a Li—Sn alloy, a Li—In alloy, a Li—Zn alloy, a Li—Ge alloy, a Li—Si alloy, or a combination thereof, but embodiments are not limited thereto, and any suitable lithium alloy may be used.

The first layer 321 may be formed of a lithium alloy, the lithium metal, or may be formed of a combination of the lithium alloys.

A thickness of the first layer 321 may be in a range of about 1 μm to about 1,000 μm, about 1 μm to about 500 μm, about 1 μm to about 200 μm, about 1 μm to about 150 μm, about 1 μm to about 100 μm, or about 1 μm to about 50 μm.

The first substrate 100 may be formed of a material that does not react with lithium, that is, a material neither forming an alloy with lithium nor a compound with lithium. Examples of the material forming the first substrate 100 may include copper (Cu), stainless steel, titanium (Ti), iron (Fe), cobalt (Co), nickel (Ni), an alloy thereof, or a combination thereof, but embodiments are not limited thereto, and any material capable of functioning as an electrode current collector may be used. The first substrate 100 may be formed of a single metal selected therefrom alone, or may be formed of an alloy of at least two different metals or a coating material. The first substrate 100 may be, for example, in the form of a plate or a foil. The first substrate 100 may be an anode current collector 21.

A second layer 322 is disposed on a second substrate.

The second layer 322 may include a carbon-based active material. The carbon-based active material may include an amorphous carbon. Examples of the amorphous carbon may include carbon black (CB), acetylene black (AB), furnace black (FB), furnace black (FB), ketjen black (KB), graphene, carbon nanotubes, carbon nanofibers, or a combination thereof but embodiments are not limited thereto, and any material classified as amorphous carbon may be used.

The second layer 322 may further include a metal alloyable with lithium. Examples of the metal alloyable with lithium may include silver (Ag), gold (Au), aluminum (Al), tin (Sn), indium (In), zinc (Zn), germanium (Ge), silicon (Si), or a combination thereof, but embodiments are not limited thereto, and any metal alloyable with lithium may be used. In the second layer 322, the metal alloyable with lithium may be omitted.

In terms of forming the second layer 322 on the second substrate, a carbon-based active material, a metal alloyable with lithium, and a binder are mixed to prepare a slurry, and the slurry may be evenly coated and dried on the second substrate 200. The second layer 322 may function as a precursor of the second anode active material layer 222.

The second substrate 200 may be, for example, formed of a material that does not react with lithium, that is, a material neither forming an alloy with lithium nor a compound with lithium. The second substrate 200 may include a material having a predetermined hardness of about 100 megapascals or greater. An example of the material of the second substrate 200 may include stainless steel, but the material of the second substrate 200 is not limited thereto, and a material which does not react with lithium, e.g., copper (Cu), titanium (Ti), iron (Fe), cobalt (Co), nickel (Ni), or combination thereof, may be used. The second substrate 200 may be, for example, in the form of a plate or a foil.

Referring to FIG. 7B, the first layer 321 and the second layer 322 are disposed to face each other, and the first substrate 100 and the second substrate 200 may be pressed together at a predetermined pressure.

For example, the first substrate 100 and the second substrate 200 may be pressed using pressing plates 1001 and 1002. The pressing plates 1001 and 1002 may be formed of a material having a predetermined hardness to facilitate the pressing of the first and second substrates 100 and 200 at a predetermined pressure. For example, a material of the pressing plates 1001 and 1002 may be stainless steel, but the material of the pressing plates 1001 and 1002 is not limited thereto.

As a result of the pressing, the first layer 321 and the second layer 322 move closer to each other and are closely contacted.

Examples of the pressing may include roll pressing, uni-axial pressing, flat pressing, warm isotactic pressing (WIP), and cold isotactic pressing (CIP), but embodiments are not limited thereto, and any suitable pressing method may be used.

A pressure applied during the pressing may be, for example, about 150 MPa or greater. A pressure applied during the pressing may be, for example, about 250 MPa or greater, or about 500 MPa or greater. A pressure applied during the pressing may be, for example, about 1,000 MPa or less. For example, the pressure applied during the pressing may be about 150 MPa to about 1,000 MPa, or about 250 MPa to about 1,000 MPa, or about 250 MPa to about 750 MPa.

A time for the pressing may be about 10 minutes or less, or about 8 minutes or less, or about 5 minutes or less, or about 1 minute or less, or about 30 seconds or less. For example, a time for the pressing may be in a range of about 5 milliseconds (ms) to about 10 minutes (min), or about 1 second to about 7 minutes, or about 30 seconds to about 7 minutes. For example, a time for the pressing may be in a range of about 2 min to about 7 min.

For example, the pressing may be performed at room temperature. For example, the pressing may be performed at a temperature in a range of about 15° C. to about 25° C., but the pressing temperature is not limited thereto, and may be in a range of about 25° C. to about 90° C., or a high temperature of about 100° C. or higher.

Referring to FIG. 7C, while the first layer 321 and the second layer 322 are pressed at a predetermined pressure, the contact layer 23, which is a third layer including the same metal as the metal in the first layer 321, is formed between the second substrate 200 and the second layer 322. Without being limited by theory, it is understood that the formation of the contact layer 23 occurs as a result of a portion of the first layer 321 moving through the second layer 322.

When the first layer 321 includes lithium metal and the second layer 322 includes a metal alloyable with lithium, the metal included in the second layer 322 may form an alloy layer during the pressing by reacting with lithium in the first layer 321 and the contact layer 23. Accordingly, the first layer 321 may be the first anode active material layer 221 including a lithium alloy, and the contact layer 23 may be a contact layer 23 including a lithium alloy.

When the metal alloyable with lithium is not included in the second layer 322, the first layer 321 may be the first anode active material layer 221 including lithium metal, and the contact layer 23 may be a contact layer 23 including lithium metal.

In the pressing of the first layer 321 and the second layer 322 with a predetermined pressure, a portion of the lithium in the first layer 321 may be injected into the second layer 322. Thus, the second layer 322 may be the second anode active material layer 222 including a carbon-based active material and lithium.

Referring to FIG. 7D, by removing the second substrate 200, the anode layer 20 is provided in which the first anode active material layer 221, the second anode active material layer 222, and the contact layer 23 are sequentially stacked on the first substrate 100 in this stated order by removing the second substrate 200.

FIG. 8 is an image that shows the anode layer 20 according to an embodiment. Referring to FIG. 8 , after the pressing of the first layer 321 and the second layer 322 at a predetermined pressure and the removal of the second substrate 200, it may be confirmed that a surface color of the anode layer 20 appears to be not black, which is the color of the second anode active material layer 222 including a carbon-based active material. In this regard, it may be confirmed that the contact layer 23 having a relatively bright color is formed on the second anode active material layer 222.

In the preparing of the anode layer 20, when the second substrate 200 is removed after pressing the first layer 321 and the second layer 322, for example, with a predetermined pressure of about 150 MPa or less, the anode layer 20 has a structure in which the first layer 321 and the second layer 322 are attached to each other, as shown in FIG. 9 . Accordingly, it may be confirmed that a surface color of the anode layer 20 appears to be black, as shown in FIG. 10 , which is the color of the second anode active material layer 222 including a carbon-based active material.

FIGS. 11 to 13 are cross-section SEM images of the anode layer 20 prepared by using the above-described exemplary method. FIG. 12 is an enlarged view of a portion of FIG. 11 , and FIG. 13 is an enlarged view of a portion of FIG. 12 .

Referring to FIGS. 11 to 13 , it may be confirmed that the contact layer 23 having a thickness less than that of the first anode active material layer 221 is formed on the second anode active material layer 222 by undergoing the preparation method described above.

Preparation of Cathode Layer

The materials constituting a cathode active material layer 12 such as a cathode active material and a binder are added to a non-polar solvent to prepare a slurry. The slurry is coated and dried on a cathode current collector 11. The obtained stack is pressed to prepare a cathode layer 10. The pressing of the stack may be performed by, for example, roll pressing, flat pressing, or isotactic pressing, but embodiments are not limited thereto, and any pressing method may be used. The pressing of the stack may be omitted. A mixture of the materials constituting the cathode active material layer 12 is compressed into the form of a pellet or stretched (molded) in the form of sheet to prepare the cathode layer 10. When the cathode layer 10 is prepared in this manner, the cathode current collector 11 may be omitted. In some embodiments, the cathode layer 10 may be used by being impregnated with an electrolyte solution.

Preparation of Solid Electrolyte Layer

A solid electrolyte layer 30 including an oxide-based solid electrolyte may be prepared by heat-treating precursors of the oxide-based solid electrolyte.

The oxide-based solid electrolyte may be prepared by contacting the precursors in stoichiometric amounts, forming a mixture, and then heat-treating the mixture. The contacting may be, for example, performed by milling such as ball milling or pulverization. The mixture of the precursors, mixed in stoichiometric amounts, is primarily heat-treated in an oxidative atmosphere to prepare a primary heat-treatment resultant. The primary heat-treatment may be performed at a temperature less than about 1,000° C. for about 1 hour to about 36 hours. The primary heat-treatment resultant may be pulverized. The pulverizing of the primary heat-treatment may be dry pulverizing or wet pulverizing. For example, the wet pulverizing may be performed by mixing a solvent such as methanol with the primary heat-treatment resultant, and milling the mixture using a ball mill for about 0.5 hours to about 10 hours. The dry pulverizing may be performed by milling the primary heat-treatment resultant using a ball mill without a solvent. A particle diameter of the primary heat-treatment resultant may be in a range of about 0.1 μm to about 10 μm or about 0.1 μm to about 5 μm. The pulverized primary heat-treatment resultant may be dried. The pulverized primary heat-treatment resultant is mixed with a binder solution and molded in the form of a pellet or may be simply pressed at a pressure of about 1 ton to about 10 tons to form a pellet.

The pellet may be subjected to a secondary heat-treatment at a temperature less than about 1,000° C. for about 1 hour to about 36 hours. From the secondary heat-treatment, a solid electrolyte layer 30 is obtained as a sintered resultant. The secondary heat-treatment may be performed at a temperature, for example, in a range of about 550° C. to about 1,000° C. The secondary heat-treatment may be performed for about 1 hour to about 36 hours. A temperature of the secondary heat-treatment is greater than the temperature of the primary heat-treatment to obtain the sintered resultant. For example, the temperature of the secondary heat-treatment is about 10° C. or greater, about 20° C. or greater, about 30° C. or greater, or about 50° C. or greater than the temperature of the primary heat-treatment. The pellet may be subjected to the secondary heat-treatment in an oxidative atmosphere, a reductive atmosphere, or a combination thereof. The secondary heat-treatment may be performed in a) an oxidative atmosphere, b) a reductive atmosphere, or c) an oxidative atmosphere and a reductive atmosphere.

For example, the solid electrolyte layer 30 including a sulfide-based solid electrolyte may be prepared by using a solid electrolyte formed of sulfide-based solid electrolyte materials.

The sulfide-based solid electrolyte may be prepared by treating starting materials with a melt quenching method or a mechanical milling method, but embodiments are not limited thereto, and any method of preparing a sulfide-based solid electrolyte available may be used. For example, when the sulfide-based solid electrolyte is prepared by using a melt quenching method, predetermined amounts of the starting materials, e.g., Li₂S and P₂S₅, are mixed into a pellet phase, reacted at a predetermined reaction temperature in a vacuum, and quenched to obtain a sulfide-based solid electrolyte. The reaction temperature of the mixture of Li₂S and P₂S₅ may be, for example, in a range of about 400° C. to about 1000° C. or about 800° C. to about 900° C. A period of time for the reaction may be in a range of about 0.1 hours to about 12 hours, or, for example, about 1 hour to about 12 hours. A temperature of the quenching may be, for example, about 10° C. or less, or, for example, about 0° C. or less, and a rate of the quenching may be in a range of, for example, 1° C. per second (° C./sec) to about 10,000° C./sec, or, for example, about 1° C./sec to about 1,000° C./sec. For example, when the sulfide-based solid electrolyte is prepared by using a mechanical milling method, predetermined amounts of the starting materials, e.g., Li₂S and P₂S₅, are mixed and reacted by using a ball mill to obtain a sulfide-based solid electrolyte. A rate and a period of time of stirring for the mechanical milling method are not particularly limited, but, when the rate of stirring is high, a production rate of the solid electrolyte increases, and, when the period of time of stirring increases, a conversion ratio from the starting materials to the solid electrolyte also increases. Subsequently, the mixture obtained from the melt quenching method or the mechanical milling method is heat-treated at a predetermined temperature, and then the resultant is pulverized to prepare a solid electrolyte in the form of particles. When the solid electrolyte has glass transition characteristics, the solid electrolyte may be changed from amorphous to crystalline by the heat-treatment.

Thus obtained solid electrolyte may be deposited by using a method known to those of skill in the art, for example, an aerosol deposition method, a cold spray method, or a sputtering method, to prepare a solid electrolyte layer 30. In some embodiments, the solid electrolyte layer 30 may be prepared by pressing a plurality of the solid electrolyte particles. In some embodiments, the solid electrolyte layer 30 may be prepared by mixing a solid electrolyte, a solvent, and a binder to prepare a mixture and then coating, drying, and pressing the mixture.

Preparation of All-Solid Secondary Battery

The anode layer 20, the cathode layer 10, and the solid electrolyte layer 30 prepared as described above are stacked in such a way that the cathode layer 10 and the anode layer 20 have the solid electrolyte layer 30 disposed therebetween to prepare a stack, and the stack is pressed to prepare an all-solid secondary battery 1.

For example, the contact layer 23 of the anode layer 20 is disposed to face a surface of the solid electrolyte layer 30, and the anode layer 20 and the solid electrolyte layer 30 are pressed at a predetermined pressure to attach the anode layer 20 to the surface of the solid electrolyte layer 30.

The pressing may be performed by, for example, roll pressing, uni-axial pressing, flat pressing, warm isotactic pressing (WIP), or cold isotactic pressing (CIP), but embodiments are not limited thereto, and any pressure application method may be used. The pressure applied in the pressing may be in a range of about 50 MPa to about 750 MPa, or about 100 MPa to about 700 MPa. A time for the pressing may be in a range of about 5 seconds to about 5 min. The pressing may be performed at a temperature, for example, in a range of room temperature to about 90° C. or about 20° C. to about 90° C. In some embodiments, the pressing is performed at a high temperature of about 100° C. or greater.

Next, the cathode layer 10 is disposed on a surface of the solid electrolyte layer 30 which is different from (e.g., opposite to) the surface on which the anode layer 20 is disposed, and the resultant is pressed with a predetermined pressure to attach the cathode layer 10 to the other surface of the solid electrolyte layer 30.

The pressing may be performed by, for example, roll pressing, uni-axial pressing, flat pressing, warm isotactic pressing (WIP), or cold isotactic pressing (CIP), but embodiments are not limited thereto, and any pressure available in the art may be used. The pressure applied in the pressing may be in a range of about 50 MPa to about 750 MPa, or about 100 MPa to about 700 MPa. A time for the pressing may be in a range of about 5 seconds to about 5 min. The pressing may be performed at a temperature, for example, in a range of room temperature to about 90° C., or about 20° C. to about 90° C. In some embodiments, the pressing is performed at a high temperature of about 100° C. or greater.

A composition and a preparation method of the all-solid secondary battery are examples of embodiments, where elements of the composition and processes of the preparation method may be appropriately modified. The pressing may be omitted.

An embodiment will now be described in more detail with reference to the following examples. However, these examples are not intended to limit the scope of the disclosed embodiment.

EXAMPLES Comparative Example 1: Anode Layer Formed of Single Lithium Metal Layer

(Preparation of Solid Electrolyte Layer/Anode Layer Stack)

A Li_(6.5)La₃Zr_(1.5)Ta_(0.5)O₁₂ (LLZO) pellet having a thickness of about 350 μm was prepared. An anode layer prepared by coating a copper (Cu) foil having a thickness of about 10 μm with a lithium (Li) metal at a thickness of about 20 μm, disposing the coated copper foil on a surface of the LLZO pellet, and applying a pressure of 250 MPa at a temperature of 25° C. to the resultant by cold isotactic pressing (CIP) to prepare a solid electrolyte layer/anode layer stack.

(Preparation of Cathode Layer)

LiNi_(0.8)Co_(0.15)Mn_(0.05)O₂ (NCM) was used as a cathode active material. Also, polytetrafluoroethylene (Teflon®; available from DuPont) was used as a binder. Also, carbon nanofibers (CNFs) were used as a conducting material. Next, the cathode active material, the conducting material, and the binder were mixed at a weight ratio of 100:2:1 to prepare a mixture. The mixture was stretched in the form of sheet to prepare a cathode active material sheet. Also, the cathode active material sheet was pressed on a cathode current collector formed of an aluminum foil having a thickness of about 18 μm to prepare a cathode layer.

The cathode active material sheet of the cathode layer was impregnated with an electrolyte solution prepared by dissolving 2.0 M of LiFSI in an ionic liquid, N-propyl-N-methyl-pyrrolidinium bis(fluorosulfonyl)imide (Pyr13FSI).

(Preparation of All-Solid Secondary Battery)

The cathode layer was disposed in a SUS (stainless steel) cap such that the cathode active material layer impregnated in the ionic liquid electrolyte solution faced upward. The solid electrolyte layer/anode layer stack was disposed such that the solid electrolyte layer was on the cathode active material layer, and the resultant was sealed to prepare an all-solid secondary battery.

The cathode layer and the anode layer were insulated with an insulator. Portions of the cathode current collector and the anode current collector were exposed to the outside of the sealed battery and used as a cathode layer terminal and an anode layer terminal.

Comparative Example 2: Anode Layer Formed of Lithium-Alloy Layer and Carbon-Metal Composite Layer

(Preparation of Solid Electrolyte Layer/Anode Layer Stack)

A Li₆₅La₃Zr_(1.5)Ta_(0.5)O₁₂ (LLZO) pellet having a thickness of about 350 μm was prepared.

A SUS304 stainless steel having a thickness of about 10 μm was coated with a carbon composite layer including a carbon-based active material and silver at a thickness of about 10 μm to prepare a first anode layer.

The first anode layer was disposed to face a surface of the LLZO pellet, and a pressure of 250 MPa at a temperature of 25° C. was applied to the resultant by cold isotactic pressing (CIP) to attach the first anode layer to the LLZO pellet. Then, the SUS304 stainless steel foil was removed from the resultant to prepare a solid electrolyte layer/first anode layer stack.

A second anode layer prepared by coating a copper (Cu) foil having a thickness of about 10 μm with a lithium (Li) metal at a thickness of about 20 μm was disposed on a surface of the solid electrolyte layer/first anode layer stack, and a pressure of 100 MPa at a temperature of 25° C. was applied to the resultant by cold isotactic pressing (CIP) to prepare a solid electrolyte layer/anode layer stack.

(Preparation of Cathode Layer and All-Solid Secondary Battery)

A cathode layer and an all-solid secondary battery were prepared in the same manner as in Comparative Example 1, except that the solid electrolyte layer/anode layer prepared as described above was used.

Example 1: Anode Layer Formed of Lithium Alloy Layer, Carbon Layer, and Lithium Alloy Layer

Preparation of Solid Electrolyte Layer/Anode Layer Stack

Carbon black, as a conducting material, and silver (Ag) nanoparticles alloyable with lithium were mixed with a binder to prepare a slurry, and the slurry was evenly coated and dried on a stainless steel foil (a second substrate). In this manner, a precursor electrode (a second layer) of an anode layer was prepared. Separately, a lithium metal electrode (a first layer) disposed on an anode current collector (a first substrate) was prepared.

After placing the prepared precursor electrode and the lithium metal electrode to face each other, a pressure of 250 MPa at a temperature of 25° C. was applied to the resultant by cold isotactic pressing (CIP) to attach the precursor electrode and the lithium metal electrode.

In the attaching process, a Li—Ag alloy layer (a contact layer) was formed between the precursor electrode and the stainless steel foil, and the lithium metal electrode was changed to a Li—Ag alloy layer (a first anode active material layer). Also, the precursor electrode was changed to a carbon layer (a second anode active material layer) including a carbon-based active material and lithium.

Then, the stainless steel was removed to prepare an anode layer in which the Li—Ag alloy layer having a thickness of about 20 μm, the carbon layer having a thickness of about 5.5 μm, and the Li—Ag alloy layer having a thickness of about 0.5 μm were sequentially stacked in this stated order on an anode current collector.

A Li_(6.5)La₃Zr_(1.5)Ta_(0.5)O₁₂ (LLZO) pellet having a thickness of about 350 μm was prepared.

The anode layer was disposed on a surface of the LLZO pellet to face the Li—Ag alloy layer having a thickness of about 0.5 μm, and a pressure of 250 MPa at a temperature of 25° C. was applied to the resultant by cold isotactic pressing (CIP) to attach the anode layer to the LLZO pellet to prepare a solid electrolyte layer/anode layer stack.

(Preparation of Cathode Layer and All-Solid Secondary Battery)

A cathode layer and an all-solid secondary battery were prepared in the same manner as described in Comparative Example 1, except that the solid electrolyte layer/anode layer stack prepared as described above was used.

Evaluation Example 1: Evaluation of Interfacial Resistance

Interfacial resistance of the full-cells prepared in Comparative Examples 1 and 2 and Example 1 were each measured.

Impedance of the pellets was measured by a 2-probe method using an impedance analyzer (Solartron 1400A/1455A impedance analyzer) with respect to the full-cells prepared in Comparative Examples 1 and 2 and Example 1. A frequency range was in a range of about 0.1 Hertz (Hz) to about 1 MHz, and an amplitude voltage was about 10 millivolts (mV).

The measurement was performed in the air atmosphere at a temperature of about 25° C. The Nyquist plots of the impedance measurement results are shown in FIGS. 14 and 15 .

Referring to FIGS. 14 and 15 , an interfacial resistance of a structure having an anode layer in which a carbon layer is in direct contact with a solid electrolyte layer (Comparative Example 2) was greater than about 2,000 Ohm·cm², but an interfacial resistance of a structure having an anode layer in which a lithium metal layer or lithium alloy layer is in contact with a solid electrolyte layer (Comparative Example 1 and Example 1) was less than about 2,000 Ohm·cm².

Evaluation Example 2: Charging/Discharging Test

Charge/discharge characteristics of the all-solid secondary batteries prepared in Comparative Examples 1 and 2 and Example 1 were evaluated by the following charge/discharge test. The charge/discharge test was performed by charging and discharging of the batteries while changing a current density under a temperature condition of about 60° C. to confirm driving characteristics of the all-solid secondary batteries in a high current density state.

As shown in FIG. 16 , a structure having an anode layer formed of a single lithium metal layer (Comparative Example 1) had short-circuits occur at about 1.0 mA/cm², and, as shown in FIG. 17 , a structure having an anode layer in which a carbon layer directly contacting a solid electrolyte layer (Comparative Example 2) had short-circuits occurred at about 0.9 mA/cm².

On the other hand, as shown in FIG. 18 , in a structure having a multi-layer structure in which a thin Li—Ag alloy layer is in contact with a solid electrolyte layer (Example 1), stable operation was possible without short-circuit occurrence until 1.8 mA/cm².

From this result, it may be evaluated as that an anode layer having a multi-layered structure in which a thin metal layer is in contact with a solid electrolyte layer may have a reduced volume change occurring during the charging/discharging, and that short-circuits of an all-solid secondary battery including the anode layer may be prevented by reducing a current being topically focused at a high current density.

As described above, according to one or more embodiments, an all-solid secondary battery and a method of preparing the all-solid secondary battery may prevent cracks of a solid electrolyte and may reduce an interfacial resistance between an anode layer and the solid electrolyte.

It should be understood that embodiments described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each embodiment should be considered as available for other similar features or aspects in other embodiments. While one or more embodiments have been described with reference to the figures, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope as defined by the following claims. 

What is claimed is:
 1. An all-solid secondary battery, comprising: a cathode; an anode; and a solid electrolyte layer disposed between the cathode and the anode, wherein the anode comprises an anode current collector; a first anode active material layer in contact with the anode current collector and comprising a first metal; a second anode active material layer disposed between the first anode active material layer and the solid electrolyte layer and comprising a carbon-containing active material; and a contact layer between the second anode active material layer and the solid electrolyte layer, wherein the contact layer comprises a second metal, and has a thickness less than a thickness of the first anode active material layer.
 2. The all-solid secondary battery of claim 1, wherein the first metal comprises lithium metal or a lithium alloy, and the second metal comprises lithium metal or a lithium alloy.
 3. The all-solid secondary battery of claim 2, wherein the first metal and the second metal are the same.
 4. The all-solid secondary battery of claim 1, wherein the thickness of the contact layer is 20% or less of the thickness of the first anode active material layer.
 5. The all-solid secondary battery of claim 1, wherein the thickness of the contact layer is about 1 nanometer to about 1 micrometer.
 6. The all-solid secondary battery of claim 1, wherein the thickness of the contact layer is less than a thickness of the second anode active material layer.
 7. The all-solid secondary battery of claim 1, wherein the contact layer does not comprise a carbon-containing material.
 8. The all-solid secondary battery of claim 1, wherein during a charge/discharge cycle, a volume change rate of the first anode active material layer is greater than a volume change rate of the contact layer.
 9. The all-solid secondary battery of claim 1, wherein during a charge/discharge cycle, a volume change rate of the second anode active material layer is greater than a volume change rate of the contact layer.
 10. The all-solid secondary battery of claim 1, wherein a volume of the contact layer after charge is about 1.5 times to about 20 times a volume of the contact layer after discharge.
 11. The all-solid secondary battery of claim 1, wherein a volume of the first anode active material layer after charge is about 1.5 times to about 500 times a volume of the first anode active material layer after discharge.
 12. The all-solid secondary battery of claim 1, wherein a volume of the second anode active material layer after charge is greater than a volume of the second anode active material layer after discharge, and the volume of the second anode active material layer after charge is about 2 times or less the volume of second anode active material layer after discharge.
 13. The all-solid secondary battery of claim 1, wherein the solid electrolyte layer comprises an oxide-containing solid electrolyte.
 14. The all-solid secondary battery of claim 1, wherein the second anode active material layer further comprises a metal.
 15. An anode-solid electrolyte sub-assembly for an all-solid secondary battery, the sub-assembly comprising: a solid electrolyte layer; and an anode on the solid electrolyte layer, wherein the anode comprises an anode current collector; a first anode active material layer in contact with the anode current collector and comprising a first metal; a second anode active material layer disposed between the first anode active material layer and the solid electrolyte layer and comprising a carbon-containing active material; and a contact layer between the second anode active material layer and the solid electrolyte layer, wherein the contact layer comprises a second metal, and has a thickness less than a thickness of the first anode active material layer. 